1. University of Rochester, Department of Physics and Astronomy.
Clumps With Self Contained Magnetic Field And Their Interaction With Shocks
Shule Li, Adam Frank, Eric Blackman
University of Rochester, Department of Physics and Astronomy.
Rochester, New York 14627
HEDLA 2012, Tallahassee, FL 1

2. Introduction
Problem of clumps and their interaction with shocks is crucial in understanding interstellar medium, supernova remnants etc. Magnetic field often plays an important role in such an interaction.
Most previous numerical studies focus on clumps immersed in a uniform background magnetic field. However, realistic clumps usually contain tangled magnetic field inside them.
Our study focus on clump's shocked behavior when there is a tangled magnetic field contained. The field's spatial distribution can be either ordered or random.
wind
ambient
clump
Problem Description 2

3. Previous Works Field aligned with shock
Magnetic field is amplified at the top of and behind the clump.
The top of the shocked clump is streamlined but there is no significant suppression on the fragmentation of the clump even for low initial β cases.
Field perpendicular with shock
Magnetic field is wrapping around the clump and gets greatly amplified due to stretching.
The shocked clump is highly streamlined and the fragmentation can be greatly suppressed even for high initial β cases.
Jones, T.W., Ryu, Dongsu, Tregillis, I.L ApJ, 473, 365
Adding radiative cooling into the simulation can further change the shocked behavior: more thin fragments, confined boundary flows, etc. (Fragile et al, 2005 ApJ 619, 327) 3

4. Lab Efforts
NLUF, Pat Hartigan et al. 4

5. AstroBEAR is a parallelized hydrodynamic/MHD simulation code suitable for a variety of astrophysical problems. AstroBEAR is designed for 2D and 3D adaptive mesh refinement (AMR) simulations, the current versioncode shows good scaling above thousands of processors. In addition, AstroBEAR comes with a number of multiphysical processes such as self gravity, thermal conduction and resistivity. 5

6. Simulation Setup Density Contrast Wind Mach Alfvenic Mach
Magnetic Beta
Crushing Time
Toroidal. Aligned
Poloidal. Perp.
Poloidal. Aligned
The simulation is set up so that the wind crossing time is shorter than the Alfvenic crossing time, the latter is shorter than the sound crossing time. We investigate the case when the contained field is ordered, as well as the case when the contained field is random. Radiative cooling is included in the simulation. 6

7. Discussion I: Clump Morphology
When the toroidal field is aligned with the shock, the clump material are compressed into a “nose cone”shape bounded by strong toroidal field. This phenomenon is similar to the “nose cone”shapes seen in MHD jet simulations.
If we add a weak poloidal field to the above toroidal field, we see a much thicker “nose cone” with a ring shaped head. It is because even a weak added poloidal field can break the axisymmetry. 7

8. Discussion I: Clump Morphology
The direction of field pinch will affect the clump's morphological evolution. 8

9. Discussion II: Mixing Ratio9

10. Discussion II: Mixing Ratio10

11. Discussion II: Kinetic Energy Transfer11

12. Discussion II: Kinetic Energy Transfer12

13. Mathematical Model
We developed a that can explain the evolution of global variables. The following expression can be used to describe how much the contained magnetic field is amplified during the compression of the shock.
EB: total magnetic energy after compression
EH: total magnetic energy after compression as if the contained field has no repelling response.
η: ratio of initial magnetic energy contained in the “perpendicular”component.
e: ratio of length on and perpendicular to the shock propagation direction.
α: ratio between contained field magnetic energy density and the shock's kinetic energy density.
μ: dimensionless factor that describes how strong the repelling force is from the contained field.
*For details, see Li et al 2012 ApJ (in preparation). 13

14. Summary
Shocked clumps with "internal" fields show rich behavior not seen in external only field simulations.
Post-shock evolution depends strongly on internal field morphology
We see sharp distinction between Toroidal/Poloidal evolution depending on alignment with shock propagation
The kinetic energy transfer and the mixing ratio depend on contained field geometry.
Simulations may provide morphological links to astrophysical clumpy environments. 14

15. Thank You ! 15